NOTES for Mat 16:20
Interpreters of the Gospel and New Testament specialists have argued much about what is usually called Jesus' "messianic secret." Why does He forbid the disciples to speak of Him as the Messiah? Why, even when Peter saw in Him precisely the Messiah, first among the apostles and probably first among people, does Jesus command him to be silent, though He acknowledges that the mystery of His messiahship was opened to Peter by the Spirit of God? The most widespread answer is quite simple: Jesus was not going to become the Messiah of popular messianic ideas.
The people expected from the Messiah quite understandable and obvious deeds: the Messiah was to restore an independent Jewish state, establish in that state laws corresponding to the Torah, and become its king. The image of the Messiah-King in popular consciousness was inseparable from the image of an earthly kingdom and a righteous ruler like David, whose descendant the Messiah was to be. Naturally Jesus was not going to do anything of the kind, and therefore "messianic" popularity among the people was of no use to Him at all. Meanwhile He could easily have gained such popularity: the miracles He performed were visible and widely known. That is why Jesus stops all rumors of His messiahship, and even forbids some to tell about the miracles He has performed.
But the matter was not only in "messianic" glory that Jesus did not need. It was also in the disciples themselves. They looked at Him, at the Messiah, much as their contemporaries did, despite all Jesus' efforts to help them see the situation differently. The same Peter, almost immediately after his messianic confession, begins to argue with Jesus when He starts telling him and the other disciples about His coming death. Even Peter, who recognized Him as Messiah, did not understand either the meaning of His path and ministry or the true significance of the Messiah and His role in the salvation of humanity. In such a state, witness is impossible.
More exactly, it is possible, but it will be witness not to the Messiah as He really is, but to one's own views and assumptions about who the Messiah is and what He should do. Therefore it was better for Peter and the other apostles to keep silent about Jesus' messiahship. The Kingdom was another matter: preaching about the Kingdom was accessible to the disciples, and at a certain point Jesus Himself sends them out to such preaching, which proved quite successful.
It was successful also because here there were no private notions, fantasies, or concepts: for the time of preaching each disciple simply became an instrument of God's direct action. But in relation to Him, to their Teacher, none of the apostles, including Peter, was ready for anything like this: too much that was human stood between Jesus and His disciples during His earthly ministry. And He commands them to be silent about His messiahship. Until the Resurrection, until Pentecost. Until the time when everything will become clear to them.